Changes in the surface properties of mammalian (rabbit) spermatozoa are being studied using a new technique for detecting changes in the ability of cells to make contact with charged electrode surfaces. The contact frequency of sperm cell suspensions is being studied as a function of cell count, ionic content of the suspending medium and charge on the electrode surface. The surface charge on the sperm cell membranes is also being measured by determining the number of zinc ions that bind to the cells. Ejaculated and epididymal sperm cells, which have different membrane charge densities, behave differently with regard to their contact frequency and sensitivity to ions. These differences are being studied to see how sperm maturation and the adsorption of substances from seminal plasma can alter the properties of the membrane. From the information about the changes in the membranes of spermatozoa at the different stages of development, we hopw to learn more about factors that influence the process of fertilization.